This invention relates to the apparatus and processes of molding and casting, particularly to molten metal casting, for such as aluminum alloy wheels, but also to other thermally or chemically hardening liquids such as solidified foodstuff, plastics, rubber or other polymers formed in molds into solid articles for such as the automotive, aerospace industries, and food technology.
For centuries, molds have been filled with molten metal to solidify into desired shapes. The processes have been adapted to other natural and synthetic hardening fluids ranging from chocolate to plastics, for example. The fluid or liquid may be poured under gravity from a vessel into an opening in the top of the mold. It may be pumped or flow under pressure into any area of the mold. The mold generally consists of a top and bottom section joined along a somewhat horizontal parting line. A cavity within the assembled mold corresponds to the desired solid shape. The opening for down pouring, or the xe2x80x9cspruexe2x80x9d, is usually cut or molded through the upper section of the mold, with some difficulties.
As most molten metals are highly reactive at the elevated temperatures required, defects often form in proportion to the speed of the filling or the height of the falling stream, both of which increase the turbulence of fluid flow and the reactive exposure; to oxygen, for instance. The mold itself may suffer erosion or liquid penetration of the mold media. The mold may even rupture under the forces applied.
Liquids usually undergo shrinkage of volume when cooled and changed into solids. This may cause defects or less than ideal mechanical properties. Additional feed liquid must remain in contact long enough to compensate for the shrinkage.
As casting has developed from an art into a science, skilled artisan foundrymen have continued to gently fill the mold cavity. They may allow time for a protective shell to solidify against the mold wall, and then fill an additional column or riser to compensate for the on-going shrinkage. The second operation is called xe2x80x9ctopping offxe2x80x9d. In this way, the mold is not exposed to high fluidity hot metal at the higher pressure. It is a time/temperature/pressure dependent physics issue. The additional pressure (as well as volume) of the higher riser(s) is necessarily added to penetrate the lattice-like dendritic structure of the solidifying metal, filling the micro voids of shrinkage with feed metal.
The artisan would also carefully cut or mold filling channels, being careful to streamline the flow as much as possible. These filling access channels, called xe2x80x9cgatingxe2x80x9d, distribute the liquid throughout the mold. Bottom filling gates have long been known as being most effective for quiescent flow. Special gating techniques within the mold, such as xe2x80x9chorn gatesxe2x80x9d (the biological name implying structure) could achieve true bottom-filling after the initial downpour through the sprue.
To control defects today, the mold is designed at great expense to accommodate fluid flow principles and to provide the risers in one filling operation. This speeds the production process but requires higher quality molds, at higher cost, capable of withstanding the early pressure of the riser height for a longer time against hot liquid (time/temperature/pressure). The molds may be of precise aggregate media formulation, or of semi-permanent material, or permanent material (i.e. metal molds). A form of xe2x80x9chorn gatexe2x80x9d is often incorporated. A disadvantage of the gating and risers is reduced cast yield.
Demand for low cost, high speed production has led to highly automated molding machines. Today, the speed of the molding operation can be very rapid, perhaps a mold every eight seconds (i.e., advanced, vertically parted, green sand molding at 500 molds per hour). Unfortunately, fluid dynamic calculations may recommend the mold be filled at a considerably slower rate, perhaps thirty seconds. Production speed or quality is often compromised, necessarily.
Mechanical devices to gently and consistently pour the metal have largely replaced the manual pouring of molds. Production speeds and quality are often improved. Radiant energy losses are exceedingly high whether manually or automatically poured. Spillage, spatter and runout also pollute the plant environment and create hazards. The heat, smoke, fumes and hazards have long made the foundry an icon of harsh industrial conditions. Filling devices that contact the mold have been proposed with the potential of substantial energy savings and quality benefits. However, these necessitate stopping or slowing the automated mold movement with loss of production speed.
Demand for increased quality in castings has led to advanced molding techniques such as low pressure permanent molding, lost foam molding and ceramic investment molding. These and other high quality systems are notoriously slow processes.
Perhaps the best combination of quality and productivity (certainly the most commercially successful) was described by the specification of U.K. Patent No. 848604 also known as the DISA (copyright)process. This is a metal casting apparatus in which green sand mold halves are arranged one behind the other, providing a succession of molds with primarily vertical parting lines. This was revolutionary. The molds are conveyed or pushed in a tightly booked line through a gravity pouring zone and sequentially filled. The sprue is molded without difficulty along the vertical parting line.
In rare instances, movable ladles or launders have been indexed to the vertical mold""s movements. This enables more suitable pouring speeds that are longer than mold cycle times, further enhancing quality and productivity. These techniques were short-lived, however, as vertical molding cycle times continued to decrease with innovations in programmable controllers and the hydraulic and pneumatic valves and cylinders of the sand compaction equipment.
In a modification of the DISA process, described in the specification of U.K. Patent No. 1,357,410, the molds are bottom filled. The velocity and pressure of the liquid metal cannot be controlled, however, to the extent required for casting of light metal alloys, such as aluminum. Vertical molding has not been widely applied to light alloys for other reasons also, discussed below. This is unfortunate for the automotive and aerospace industries. High integrity aluminum castings are critically needed. Much of the huge demand (for instance: automotive alloy wheels) has been met by low pressure permanent molding (LPPM) at slow speed and high operational cost.
The foam molding casting method has high capability for aluminum casting. It comprises embedding a pattern of foam plastics material (i.e. expanded polystyrene) or other replaceable material in loose sand. The process is quite slow compared to vertical green sand molding.
The foam molding technique also suffers from the disadvantage of sporadic filling defects. Further attempts to provide a method of casting whereby this problem is reduced as disclosed in the specification of U.S. Pat. No. 4,693,292, which comprises the step of feeding molten metal generally upwardly against the force of gravity. This is again a form of the ancient artisan""s prior art xe2x80x9chorn gatexe2x80x9d which was called a xe2x80x9criser tubexe2x80x9d (not to be confused with the traditional elevated shrinkage xe2x80x9criserxe2x80x9d). These and other so-called xe2x80x9ccounter gravity processesxe2x80x9d are exceedingly slow by waiting for solidification before the next operation. The mold must stay connected to a metal source for a time sufficient for the casting(s) within to at least become self-supporting. For high rates of productivity, multiple casting stations and sets of expensive molds are necessary.
The desired direction of solidification is always toward a source of liquid feed. In bottom filling, this is initially from the coldest liquid metal at the top of the mold towards the hot metal at the bottom. Natural convection within the mold, however, attempts to move the hot metal to the top of the mold over a period of time. This changes the direction of solidification to be more like a top filled system, the degree dependant also on alloy conductivity. Counter gravity casting may thus cause shrinkage porosity.
The specification of U.S. Pat. No. 5,477,906, disclosed a thermal extraction technique using a seal to isolate the mold from the liquid metal source and allow the mold to be moved more quickly, providing a more efficient use of the casting station. A solidified protective shell against the mold wall is still required before movement.
A variation of the low pressure casting method involves a small secondary metal source in the mold cavity itself With the secondary metal source, the mold can be inverted and then disconnected from the primary metal source. The casting is allowed to solidify elsewhere whilst being fed from the secondary metal source. Inverting equipment is required.
The known Cosworth (copyright)process as disclosed in the specification of U.S. Pat. No. 4,733,714 utilizes such an inverting, or rotating, operation and effectively takes advantage of the fluid flow and solidification science discussed above. The Cosworth process achieved improved properties of casting by pressurized filling and feeding. The method dramatically slows the production speed however and it is not applicable to the high speed vertical molding process. It appears suitable only for light metal alloys.
These new processes are not well adapted to the commercially important ductile iron and compacted graphite metals. Being highly reactive, this metal does not adapt well to the discussed prior art. Slag inclusions and shrinkage defects are common. Horizontal green sand molding is still the best process for ductile iron, particularly if using the xe2x80x9cinmoldxe2x80x9d (registered trademark) treatment process (discussed below).
The Danish equipment manufacturer, DISA(copyright), has commercially proposed a mold sealing technique with low level filling of the vertical green sand molds by a pump. This was an effort to support aluminum casting. Unfortunately, under gravity or low pressure filling, molds are required to be permeable for escape of air and reaction products, especially to allow high speed filling. This requires a coarse mold media (i.e. coarse sand) or vents in the mold. Coarse sand, however, does not cool the metal rapidly enough to obtain the fine microstructure required for automotive and aerospace aluminum.
A much finer sand is also required to resist liquid penetration of the mold wall, if any appreciable pressurized shrinkage feeding is applied. The resultant low permeability of fine sand and the mechanical delay of contacting and disconnecting from the mold (also inserting the seal) would cause the vertical green sand molding system to lose it""s high speed advantage.
Aluminum alloy wheels are an example of an enormously energy consumptive cast product. Huge worldwide demand for this casting, other metal castings, and other thermosetting liquids screams for a high production, high integrity, energy efficient process. In the case of aluminum wheels, high speed vertical green sand molding could be the method of choice if quality were enhanced comparatively with low pressure permanent molding. Contact methods of filling, as DISA has proposed, have the potential to eliminate radiant energy losses of exposed transfer operations and open sprues. The higher net yield of contact filling saves melting energy also.
If the vertical green sand mold could withstand highly pressurized shrinkage feeding, without rupture or penetration of the sand, high density cast wheels would be possible. If the mold media were finer, the castings would more rapidly solidify, producing the desired dendrite arm spacing and fine microstructure needed.
Another aspect that must be addressed in a comprehensive solution is the need for late addition of catalyst or modifiers in casting processes. For best grain structure a late sodium or titanium addition might be injected into aluminum alloy for automotive or aerospace castings. Liquid chemicals might be solidified or modified by a catalyst addition.
These additions typically complete their reaction quickly, or fade quickly. Pouring must be quickly accomplished. In U.S. Pat. No. 3,703,922, concerning the treating of iron with magnesium or rare earths, it was proposed that treating adjacent to or in the mold itself would minimize fade, among other benefits. This treatment idea has the most cost effective and environmental benefit potential. However, the process has not adapted well to vertical molding or to bottom filling. Reaction products can be minimized with less exposure to oxygen (being in the closed mold) but entrapment of the dross is still a challenge. Slow pouring speeds are required for cleanliness.
We are thus left with the choice of high quality casting or high speed casting, but not both. The high quality processes such as investment casting and lost foam are cost and labor intensive. Low Pressure Permanent Molding and the Cosworth Process involve delays for coupling, cooling, inverting, uncoupling, and other handling operations. Expensive mold media are also required. The high production processes, such as vertical green sand, require rapid and turbulent pouring or filling times to keep pace with mold speed. Pressurized solidification is impossible with high speed. The required permeable green sand mold media have inadequate heat absorption and inadequate resistance to fluid penetration. Bottom or side filling of any of these processes is a cumbersome project and challenges our desire for late treatments in the mold.
The two primary objects of this invention are to fill high speed molds with high quality results and secondly, to automate high quality casting techniques for high speed processing. Several necessary related objects and advantages result:
1. Vertical green sand molding machines with finer, more capable sand may run at optimum speed while the molds are gently, and slowly, bottom or side filled.
2. Vertical green sand molds may be highly pressurized without rupture or penetration.
3. High density, fine structure aluminum castings may be cast in high speed vertical green sand.
4. Aluminum alloy wheel production is enabled and vastly increased in vertical or horizontal green sand.
5. Lost foam molds may be continuously bottom filled, automating the process.
6. Investment castings, such as aerospace turbine blades may be rapidly cast in an automated fashion.
7. Various types, sizes and shapes of molds may be combined in one continuous, pressurized, filling and feeding machine.
8. A technique for multiple filtration of mold filling fluid is provided.
9. A liquid (particularly, a molten metal) treatment or modification method results, including precise production of ductile iron and compacted graphite iron castings.
10. A mold support device is embodied and suitable for reinforcing conventionally poured molds.
11. Energy is conserved by a greatly reduced exposure of molten metal.
12. A safer and cleaner environment results by eliminating metal splash in pouring.
13. Small and large foundries become more flexible and competitive.
14. Plastic and rubber molding operations can use the filling and feeding system, for such as tires.
15. Food processing in molds is automated and increased with cost and energy savings.
16. New filter cloth designs useful for sealing processes and liquid modification have resulted.
17. A sonic metal height control system has resulted.
The solutions to the disadvantages of the prior art and to other difficulties not discussed, have not been reached prior to my comprehensive invention, as they come only by simultaneous application of certain principles producing a unity of invention. The following examples are not exhaustive and not always mandatory. The unity is not immediately obvious but will become so in study of the forthcoming drawings and detailed description.
1. Fluids are incompressible. When fluids are contained and displacement is not allowed, massive objects, such as molds, may float freely.
2. Sand and other mold media is of less density than liquid molten metal. When submerged in a bath of liquid metal, buoyant forces are generated against molds.
3. Filter cloth may be designed to allow fluid flow in one axis perpendicular to the cloth but effectively seal flow laterally and longitudinally. Cloth may be laminated to achieve other properties.
4. Cascading molten metals generate slag. Bottom filling of castings thus produces cleaner castings. Slow filling and/or filtering produces cleaner castings.
5. Increased pressure of filling aids the filling or casting of thin sections by overcoming the surface tension of liquid.
6. The surface tension of fluids limits the ability of liquids to penetrate a potential path of escape. Increased pressure is thus required to experience runout failure through a seal. As the potential path thins, the surface area to volume ratio increases, freezing the molten metal and forming a solid seal to any further leakage.
7. Modern green sand (clay bound) molding machines can produce molds of exceptional and uniform, density, hardness and strength. Yield strength exceeds 5 psi, or 3.45 N/cm2. A pressure head of liquid metal against a mold may approach the molding squeeze pressure without yielding of the mold.
8. Under hydraulic ramming of green sand, the phenomenon of grain-to-grain contact can occur. In vacuum bound molds, or vibration packed, loose sand molds, the ultimate grain-to-grain contact occurs, making molds as hard and rigid as stone. By restraining shear in all directions, such molds may push one another in an unlimited line traveling in one direction.
9. A height of liquid produces a pressure proportional to the density and height of the liquid column above a point. Additional pressure may be applied to the column of liquid by air or inert gas pressurization for filling and/or feeding.
10. Vacuum may lift a column of liquid proportionately to the density of the liquid under influence of gravity and the percentage of atmospheric pressure evacuated. Forty to fifty percent evacuation is within the practical limits of industrial vacuum and is equivalent to, approximately xe2x88x927 psig or xe2x88x924.83 N/cm2.
This approximately equals a vacuum of:
14 inches Hg (35 cm);
27 inches Fe (69 cm), and;
88 inches Al (224 cm).
xe2x80x83Compact vacuum and pressure controlled furnaces may be built handling metals within these height limits.
11. Hot expanded molten metals contract and shrink as they cool and solidify. This contraction must be supplemented with additional feed metal.
12. Castings cool and freeze from the surface inward, initially producing a solid shell over a core of liquid metal. Solid dendrites grow from the surface into the centerline liquid blocking the flow path of additional feed metal. Increased pressure during solidification will drive feed metal through the structure, producing denser, stronger castings.
13. Liquid metal can penetrate sand grains under excessive pressure, creating a poor surface finish in a casting. Limiting the pressure until a solid shell forms will produce a better surface. Very fine mold media produces a better surface finish and more rapid solidification. Finer media requires pressurized filling due to lower permeability.
14. In casting, solidification shrinkage feeding is always partially accomplished through the gates used for filling the mold. Specialized gates called risers (rising above the casting) normally complete the feeding. In the subject invention, gating and risering are synonymous and usually referred to as gates 108.
15. Ultrasonic waves may pass through porous media to a sufficient degree to back-reflect from the boundaries of denser material beyond. The travel time of the wave may be measured electronically and be converted by an algorithm into precise measurement of material thickness or depth.
16. Data loggers may continuously monitor molten metal depth and pressure, within and around or above the molten metal to regulate applied pressure. Computer processors may continuously analyze such data to adjust for changing conditions and accurately control a process.
The apparatus and processes of the subject invention work in unity or separately to continuously fill any and all types of molds at selectable and controlled pressure(s) and filling speed(s). The filling is independent of mold production cycle time. In the preferred embodiments, pressurized, shrinkage feeding of moving, solidifying castings is a second operation that produces high integrity castings. Liquid treatment methods such as filtration, alloying and modification are accommodated and improved in association with the process.
Use of the machine and process is applied to various ferrous and non-ferrous cast articles including aluminum alloy wheels and engine cylinder heads. The invention applies to any hardening fluid or liquid element or compound, molded or cast for any industry or use, with quality, cost, environmental and energy conservation benefits.